Nuclear fuel containers are subject to leakage failures attributable to corrosion, in particular a phenomenon defined in this technology as stress corrosion cracking, an occurrence which is primarily induced or accelerated by abrupt or rapid reactor power increases. Composite nuclear fuel containers, or fuel elements, have been introduced and frequently employed in power generating, commercial water cooled nuclear fission reactor plants to cope with this shortcoming of stress corrosion cracking. Composite nuclear fuel containers comprise a generally conventional tubular container, constructed of a zirconium alloy, stainless steel, aluminum, or other suitable alloy of the art, provided with an internal lining which functions as a protective barrier, and is composed of a metal having increased resistance to intergranular stress corrosion cracking, or other forms of destructive attack. The barrier linings of the art comprise a variety of metals and alloys, including zirconium metal of substantial purity, for example less than about 5000 parts per million impurities, copper, molybdenum, tungsten, rhenium, niobium and alloys thereof. Examples of such protective metal barrier linings for nuclear fuel tubular containers comprise U.S. Pat. No. 4,200,492, issued Apr. 29, 1980; U.S. Pat. No. 4,372,817, issued Feb. 8, 1983; U.S. Pat. No. 4,390,497, issued Jun. 28, 1983; U.S. Pat. No. 4,445,942, issued May 1, 1984; U.S. Pat. No. 4,659,540, issued Apr. 21, 1987; U.S. Pat. No. 4,942,016, issued Jul. 17, 1990; and U.S. Pat. No. 4,986,957, issued Jan. 22, 1991.
Typical composite fuel containers of the art, comprising a tubular containing having a metal liner providing an internal barrier layer metallurgically bonded to its inner surface, are produced by inserting a section of a large diameter, hollow liner stock unit in close fitting intersurface contact into and through the length of a section of a large diameter tube stock. This composite assembly of large diameter section of tube stock with inserted liner stock is then subjected to a series of circumference reductions with each reduction accompanied by a following heat annealing to reduce the hardness imposed by the cold work distortion of the diameter reduction. Various methods can be used to metallurgically unite the tube and liner components, including explosive bonding, heating under compressive loading to cause diffusion bonding, and extension of the assembly. Detailed examples of methods for producing such composite constructed nuclear fuel containers are given in U.S. Pat. No. 4,390,497, issued Jun. 28, 1983; U.S. Pat. No. 4,200,492, issued Apr. 29, 1980; and U.S. Pat. No. 4,372,817, issued Feb. 8, 1983.
In addition t the conventional annealing heat treatments for the purpose of relieving reduction compression induced stresses in the metal of the reduced composite tube and liner unit, it has become a common practice in this field to subject such nuclear fuel containers to specific modifying heat treatments to enhance or optimize a critical property thereof such as corrosion resistance or ductibility as a means for improving the fuel elements continuing durability. For instance, it is well known to heat treat zirconium metal and its alloys, or components formed thereof, up to a temperature of above the alpha microcrystalline phase of the particular metal composition, or to the alpha plus beta or beta microcrystalline phase, followed by rapid cooling to preserve significnat or critical aspects of the resulting heat induced microstructure state. Such heat treatments are disclosed in detail in the prior art, for example U.S. Pat. No. 2,894,866, issued Jul. 14, 1959; U.S. Pat. No. 4,390,497, issued Jun. 28, 1983; U.S. Pat. No. 4,238,251, issued Dec. 9, 1980; and U.S. Pat. No. 4,576,654, issued Mar. 18, 1986.
Temperatures for producing the various potential microstructure changes and accompanying property modifications disclosed in the literature typically depend upon the exact composition of the metal or alloy, and are essentially a unique condition for each different metal or combination of alloying ingredients. Thus if the temperature conditions to achieve or optimize a particular characteristic in a specific composition is no readily available in the literature, it can be ascertained empirically, note for example U.S. Pat. No. 2,894,866, issued Jul. 14, 1959; and U.S. Pat. No. 4,238, 251, issued Dec. 9, 1980.
The disclosures and contents of all the aforesaid U.S. Letters Patent, and the references cited therein, are incorporated herein by reference.